Although prior research has demonstrated that relationship characteristics and partner type (e.g., casual vs. primary sexual partner) are associated with HIV risk behavior, the role of relationship dynamics in sexual risk behavior has only begun to be examined. This grant application requests support for a 4 1/2-year project to examine relationship dynamics within the heterosexual involvements of men and women of reproductive age at high-risk for HIV infection. The overall objective of the proposed research is to improve understanding of the influence of relationship dynamics on sexual risk perceptions, intentions, and behaviors. Relationship dynamics are defined as factors present at a specific time within a given dyadic involvement that emerge as a result of that involvement (e.g., level of commitment, sexual decision-making). The specific aims of the study are to 1) track transitions in partnerships and associated relationship dynamics over time among a sample of high-risk men and women;2) explore partnership-specific relationship dynamics associated with initiation of concurrent sexual partnerships and the duration of concurrence, overall and by gender;3) assess the differential predictive abilities of partnership-specific factors with respect to individual-level sexual risk perceptions, intentions and behaviors, overall and by gender;and 4) determine whose condom use intention best prospectively predicts condom use among dyad members who differ in their expressed intentions, using data from both dyad members. A longitudinal study of 528 individuals with four distinct measurement occasions at four-month intervals will be conducted. The study will track high-risk men and women as they initiate, maintain, and/or terminate sexual relationships. At Time 3, an estimated 150 men and 150 women who are the sexual partners of members of the initial sample will also be enrolled in the study. At Times 3 and 4, data will be collected from both members of the dyad separately and together and analyzed using the Actor-Partner Interdependence Model (APIM). Findings will have significant implications for the design of interventions for reducing the risk of HIV infection among individuals in close relationships by addressing the dynamics of sexual partnerships.